1. Field of the Invention
The present invention relates to a vertical transition device for differential stripline paths and more particularly to a vertical transition device for connecting paths on a horizontal plane with paths on another horizontal plane. The present invention also relates to an optical module incorporating the vertical transition device.
2. Prior Art
Optical modules, which are devices used for transmitting and receiving optical signals through optical fibers, are needed to enhance transmission speed of data while it should be downsized. On account of such demands, developed was a type of optical module incorporating an electrical/optical converting element such as a semiconductor laser diode, an amplifier for actuating the E/O converting element, an MUX (multiplexer), a DEMUX (demultiplexer), and other suitable elements integrally.
It is necessary to exchange various sorts of signals including lower frequency signals and radio frequency signals between the structural elements of the module. Therefore, in order to minimize influences of exterior noises and inequality of power supply voltage, this type of optical module is usually provided with a pair of differential paths for propagating differential signals.
A package architecture of the module may comprise a multilayered path arrangement including a plurality of dielectric materials, such as ceramic substrates, arranged in layer, and signal paths and power supply paths formed on or between the dielectric materials. To assemble such a package architecture of an optical module with a high packing density from such multilayered path structures, it is preferable to utilize a vertical transition device wherein differential microstrip lines and differential triplate lines on both sides of a dielectric layer are interconnected by vertical via-holes.
FIGS. 9 through 11D show a conventional vertical transition device for a stripline path. FIG. 9 is a see-through perspective view showing the vertical transition device. FIG. 10 is a vertical cross sectional view taken along line X-Xxe2x80x2 in FIG. 9. FIG. 11A is a top view of the vertical transition device. FIG. 11B is a horizontal sectional diagram of the vertical transition device taken along plane A in FIG. 9. FIG. 11C is a horizontal sectional diagram of the vertical transition device taken along plane B in FIG. 9. FIG. 11D is a horizontal sectional diagram of the vertical transition device taken along plane C in FIG. 9.
As shown in the drawings, the vertical transition device comprises dielectric layers 1, 2, and 3, a microstrip line 4, a triplate line 5, a signal via-hole 6, ground planes 7 and 8, and three matching via-holes 9. The matching via-holes 9, which connect the ground plane 7 with the ground plane 8, are arranged in the vicinity of the signal via-hole 6 and equally apart from the signal via-hole 6, so as to form a coaxial path structure. The signal via-hole 6 is connected at both ends with the microstrip line 4 and the triplate line 5.
Adjusting the distance between the signal via-hole 6 and the matching via-holes 9 results in a change of the impedance of the coaxial path structure. It means that it is possible to match the impedance of the coaxial path structure with the characteristic impedance of the microstrip line 4 and the triplate line 5 by a prior experiment or a simulation. Thus, a suitable vertical transition device in which impedance matching is accomplished for a stripline path can be manufactured.
In an application of the above-described vertical transition device to an optical module having a pair of differential paths, two vertical transition devices are interposed in the differential paths, respectively. In other words, a conventional vertical transition device for differential stripline paths comprises a pair of this type of vertical transition devices.
With such a structure, the conventional vertical transition device for a stripline path involves problems that will be described next.
FIG. 12 is a conceptual diagram showing a cross section of differential microstrip paths taken along a plane perpendicular to the signal propagation direction, and showing lines of electric forces. Sign S indicates the distance between the microstrip lines constituting the microstrip paths while sign W indicates the width of each microstrip line. Differential microstrip paths has a propagation mode wherein an electric field between the adjacent microstrip lines and electric fields between the ground plane and the microstrip lines are coupled with each other. It is a merit of the differential microstrip paths to lessen the influence of exterior noises or disturbances upon the subject electric signals. In order to bring out the merit, it is preferable that the distance S is narrow for concentrating the field intensity at the region between the microstrip lines.
However, although the above-described conventional aggregation of two stripline vertical transition devices is utilized in differential paths, the distance between microstrip lines is too long to couple electric fields together. This seriously impairs the merit of the differential paths. In addition, such an aggregation is complicated and large too much, and the provision of a plurality of matching via-holes 9 leads a further enlargement and a further complication of the resultant vertical transition device.
Accordingly, it is an object of the present invention to provide a vertical transition device accommodated to differential stripline paths, having a simpler construction without use of matching via-holes.
It is another object of the present invention to provide an optical module incorporating the vertical transition device.
In accordance with an aspect of the present invention, there is provided a vertical transition device for differential stripline paths, comprising differential microstrip paths and differential triplate paths. The differential microstrip paths include a first dielectric layer, a second dielectric layer, a first ground plane interposed between the first and second dielectric layers, and first and second microstrip lines disposed on a surface of the first dielectric layer opposing to the first ground plane, the microstrip lines and the first dielectric layer causing an electric field coupling for propagating differential signals. The differential triplate paths include a third dielectric layer, a second ground plane disposed on a surface of the third dielectric layer, and first and second triplate lines disposed between the second and third dielectric layers, the triplate lines and the first and second dielectric layers causing an electric field coupling for propagating the differential signals. The vertical transition device further comprises a first via-hole for connecting an end of the first microstrip line with an end of the first triplate line, a second via-hole for connecting an end of the second microstrip line with an end of the second triplate line, and an aperture formed in the first ground plane, the first and second via-holes are located within the aperture, so that the via-holes are isolated from the first ground plane.
The distance between the first and second via-holes may be longer than the distance between the first and second microstrip lines.
Preferably, the distance between the first and second via-holes is selected such that a return loss is desirable.
Alternatively, the distance between the first and second via-holes may be substantially equal to the distance between the first and second microstrip lines.
In a preferred embodiment, the diameter of the first and second signal via-holes is less than 0.1 mm.
Preferably, the diameter of the first and second signal via-holes is selected such that a return loss is desirable.
In accordance with another aspect of the present invention, there is provided a vertical transition device for differential stripline paths, comprises first differential triplate paths and second differential triplate paths. The first differential triplate paths include a first dielectric layer, a second dielectric layer, a first ground plane disposed on a surface of the first dielectric layer, a second ground plane disposed on a surface of the second dielectric layer, and first and second triplate lines interposed between the first and second dielectric layers, the first and second triplate lines and the first and second dielectric layers causing an electric field coupling for propagating differential signals. The second differential triplate paths include a third dielectric layer, a fourth dielectric layer, the second ground plane interposed between the second and third dielectric layers, a third ground plane disposed on a surface of the fourth dielectric layer, third and fourth triplate lines disposed between the third and fourth dielectric layers, the third and fourth triplate lines and the second and third dielectric layers causing an electric field coupling for propagating the differential signals. The vertical transition device further comprises a first via-hole for connecting an end of the first triplate line with an end of the third triplate line, a second via-hole for connecting an end of the second triplate line with an end of the fourth triplate line, and an aperture formed in the second ground plane, the first and second via-holes are located within the aperture, so that the via-holes are isolated from the second ground plane.
Preferably, the distance between the first and second via-holes is substantially equal to the distance between the first and second triplate lines or to the distance between the third and fourth triplate lines.
In accordance with another aspect of the present invention, there is provided an optical module comprising an optical semiconductor device and any one of the above-described vertical transition devices for propagating differential signals to or from the optical semiconductor device inside the optical module.